Shimano XTR M980 pedal problem- Stiff NDS pedal

My XTR 980 NDS pedal frequently become very stiff at the pedal-axle interface (cleat engagement is fine). I have taken it apart, replaced the small ball bearings, properly regreased, properly tensioned the bearings/ tightened the nuts several times and each time it is fine for a ride or two before becoming stiff again.

I can only think that as the pedal body is turning, it is somehow tightening the NDS nuts but I have payed attention to making sure both nuts are correctly tightened with no change to the stiff pedal a ride or two later. There is a good chance I am doing it wrong though! I have been adjusting the first nut (nearest the bearings) so the slack is just taken up and everything can spin freely,then tighten the second nut to firmly hold the first in place. I forget the exact torque but no more than 4 or 5Nm (whatever the instructions say) on the second nut. The one thing I haven't done is use any Loctite- I suppose this might help???